The structual organization of human plasma lipoproteins is being investigated employing natural abundance carbon-13 nuclear magnetic resonance spectroscopy. Well-resolved spectra of all classes of native lipoproteins have been obtained and assigned. Values of T1 for several resonances of lipid carbon atoms have been measured; these reveal a fluid structure for lipoproteins but one in which significant restrictions on segmental and rotational mobilities of lipid molecules exist. Similar spectra have been recorded for high-density and low-density lipoproteins following trypsinization. Spectral changes reveal some structural reorganization of the lipoproteins. The intermediate-density lipoproteins of type III hyperlipoproteinemia patients have been studied. The structural organization of these resembles that for the very-low-density lipoproteins. BIBLIOGRAPHIC REFERENCES: A.R. Goldhammer, M.K. Jain, and E.H. Cordes, Phospholipases. III. Effects of Ionic Surfactants on the Phospholipase-catalyzed Hydrolysis of Unsonicated Egg Lecithin Liposomes. J. Membran Biol., 23, 293 (1975). B.K. McQuoid, A.R. Goldhammer, and E.H. Cordes. Secondary Valnce Force Catalysis. XVI. Melittin-catalyzed Hydrolysis of p-Nitrophenyl Dodecanoate. J. Org. Chem., in press (1976).